Open Collector Outputs are increasingly common in digital chip design, operational amplifiers and micro-controller (Arduino) type applications, for either interfacing with other circuits or for driving high-current loads such as indicator lamps and relays which maybe incompatible with the electrical characteristics of the control circuit. 

We know from our previous tutorials that a bipolar junction transistor, whether it is an NPN type or a PNP type, is a 3-terminal device. These three terminals are identified as being the Emitter, the Base, and the Collector. We can use bipolar transistors to operate as either an Amplifier, that is the output signal has a greater amplitude than the input signal, or more commonly, as a solid state “ON/OFF” type electronic switch.